This is an inner join, and is an intersection of the data in the tables. This is shown in the image below.
Careful here. An Inner Join and an Intersection are not the same thing. Unlike a JOIN, an Intersection returns a set, duplicates are removed. Joins return a multiset, duplicates are allowed. I struggled with this early on.
One final note on performance. While outer joins can be very helpful and handy, they do come with a performance penalty of sorts.
It depends here. OUTER <> lesser performance. I've run into a number of situations where switching to an OUTER Join then filtering the NULLs improved performance.
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."
-- Itzik Ben-Gan 2001